The present invention generally relates to branch joint box used for branch connection of wiring harnesses of a motor vehicle and more particularly, to a branch joint box in which a plurality of connector blocks each having a plurality of terminals are formed integrally with a bottom wall such that the terminals are connected to each other by a printed circuit board.
A branch joint box in which a printed circuit board is used for connecting terminals to each other is known from, for example, Japanese Patent Laid-Open Publication Nos. 61-126783 (1986) and 63-265487 (1988) as shown in FIGS. 1 to 3. In FIG. 1, a plurality of connector blocks 3 are provided on an upper face of a bottom wall 2 of an upper casing 1 of the branch joint box. In each of the connector blocks 3, a plurality of pin type terminals 4 are erected vertically on the upper face of the bottom wall 2. A printed circuit board 5 for connecting the terminals 4 of the connector blocks 3 is secured to a lower face of the bottom wall 2 by screwing or resinous caulking.
In FIG. 2, the connector blocks 3 are molded integrally with the bottom wall 2 of the upper casing 1 by resin and the bottom wall 2 serves also to support the terminals 4 of the connector blocks 3. Each of the terminals 4 is inserted through the bottom wall 2 and the printed circuit board 5 so as to project out of a lower face of the printed circuit board 5 and a distal end 4a of the terminal 4, which projects out of the lower face of the printed circuit board 5, is fixed to a conductive pattern of the printed circuit board 5 by a solder portion A. The printed circuit board 5 is generally produced by impregnating a plain paper sheet with phenolic resin and hardening the impregnated paper sheet to a thick platelike shape. Meanwhile, the upper casing 1 is generally made of polypropylene or the like and is reinforced by talc, etc.
In addition to the above mentioned known branch joint box shown in FIGS. 1 to 3, FIG. 4 shows another branch joint box in which a plurality of connector blocks 3 made of resin are provided separately from a thick printed circuit board 5 so as to be secured to the printed circuit board 5 by screws, etc.
In the known branch joint box of FIGS. 1 to 3 in which the printed circuit board 5 obtained by hardening the impregnated paper sheet to a platelike shape is secured to the bottom wall 2 of the resinous upper casing 1 such that wiring is performed among the terminals 4, coefficient of thermal expansion of the resinous upper casing 1 is quite higher, for example, 5 to 10 times higher than that of the printed circuit board 5. Therefore, even when the printed circuit board 5 is hardly deformed by temperature changes, the resinous upper casing 1 is subjected to large thermal expansion or contraction, so that the following inconveniences (1) and (2) are incurred.
(1) Due to difference in thermal deformation between the upper casing 1 and the printed circuit board 5, terminal holes 7 of the bottom wall 2 are brought out of alignment with terminal holes 8 of the printed circuit board 5, so that the terminals 4 are slanted as shown by the imaginary lines in FIG. 3 and thus, large tensile stress or compression stress is applied to the solder portions A. Upon repetitive application of such stress to the solder portions A, cracks are produced on the solder portions A, thereby resulting in a possibility of improper electrical conduction.
(2) Furthermore, when the pin type terminals 4 are slanted as described above, the terminals 4 deviate greatly from terminal receiving bores of a connector of a wiring harness, respectively and thus, the connector of the wiring harness cannot be connected to the terminals 4 smoothly.
Meanwhile, also in another prior art branch joint box shown in FIG. 4, the same problems as described above arise.
Moreover, a branch joint box in which a flexible sheet is used for the printed circuit board has been also developed as disclosed in, for example, Japanese Patent Laid-Open Publication No. 1-202109 (1989). In this prior art branch joint box, bus bars are also used and bent terminals are formed integrally with the sheetlike printed circuit board. However, in this prior art branch joint box, since the sheetlike printed circuit board is employed as direct contacts for the terminals, the printed circuit board should be reinforced by using a reinforcing plate or the like, so that assembly of the prior art branch joint box is time-consuming. Furthermore, mechanical strength of the terminals in this prior art branch joint box is lower than that of the construction employing soldering for securing the terminals to the printed circuit board.